In the recording/reproducing system for recording/reproducing digital information with respect to recording medium, e.g., magnetic disc by the magnetic head, the magnetic disc is rotated at a high speed at the time of operation, whereby recording/reproduction operation is carried out in the state where a floating slider (floating head) including the magnetic head mounted thereat is caused to slightly float spaced above the magnetic disc surface.
In this case, in the floating head caused to float, the floating distance must be reduced to below a maximum limit in order to suppress spacing losses. In the case of the floating head of small floating quantity type, the floating quantity is set to about 50 nm. When projections higher than this floating quantity exist on the magnetic disc surface, the magnetic head collides with the projections, resulting in breakage of the magnetic disc or the magnetic head.
Therefore, in aluminum substrates which are widely used as the magnetic disc substrate, turning (lathe) machining or pressure annealing is implemented to carry out finishing off undulations or uneveness of the principal surface so as to remove 20 .mu.m or less. Thereafter, a coating by electroless deposition (plating) layer of Ni--P alloy is performed so that its thickness is equal to about 30 .mu.m to provide necessary hardness to the surface. The hardened surface is thereafter subjected to a grinding operation so that the average surface roughness is equal to about 0.02 .mu.m and thickness is equal to about 15 .mu.m, whereby mirror finish is provided.
However, such processing method has the problem that productivity is remarkably inferior (poor), so not only there results increased manufacturing cost, but also it is impossible to provide in advance uneven portions at the portion where servo mark is to be formed in correspondence with, e.g., realization of high recording density.
It is also well known to mold optical disc substrates. In the case where plastic material is used, both disc substrate having no uneven portion and disc substrate on which uneven portions are formed can be easily molded by selection of metal mold or stamper at the time of injection molding.
As the magnetic disc substrate of resin a, substrate using thermoplastic norbornene resin is disclosed in the Japanese Patent Application Laid Open No. 153060/1995 publication. When plastic material used for the magnetic disc substrate is, e.g., polycarbonate or polymethyl methacrylate, etc. as used in the optical disc substrate, deformation by moisture absorption is large. For this reason, it cannot be necessarily said that such material is suitable. In addition, in the case of polymethyl pentene or polystyrene, etc. even if that material is plastic material of low moisture absorption, the substrate is deformed after molding by crystallinity defects or the heat-resistance may be insufficient. This is not preferable.
On the contrary, in the case of the magnetic disc substrate using thermoplastic norbornene resin, deformation by moisture absorption or crystallinity hardly takes place, and heat resistance is also sufficient.
However, as the result of studies by inventors of this invention, even in the case of the magnetic disc substrate of resin using this thermoplastic norbornene resin, although surface roughness of metal mold or stamper used in the injection molding is controlled so that it is in correspondence with the same degree as that of the magnetic disc substrate of aluminum, it has been found that projections higher than floating quantity of the magnetic head is unforeseenly formed on the surface.
In the magnetic disc using magnetic disc substrate having such projections, particularly in the case where floating quantity of the floating head is reduced to the limit, reliability is damaged to much degree.